While semiconductor integrated circuits are used in a wide variety of applications, an increasingly finer circuit design is required for such goals as a higher density of integration and a saving of power consumption. In liaison with the demand, the circuit-forming lithography involving exposure through a photomask uses a light source of shorter wavelength in order to produce a finer image. In the advanced lithography process for current use on a commercial basis, the light source for exposure has shifted from KrF excimer laser light (248 nm) to ArF excimer laser light (193 nm).
The lithography using ArF excimer laser light of greater energy was found to cause damages to the mask, which were not found with KrF excimer laser light. One problem is that on continuous use of the photomask, foreign matter-like growth defects form on the photomask. These growth defects are also known as “haze”. The source of haze formation was formerly believed to reside in the growth of ammonium sulfate crystals on the mask pattern surface. It is currently believed that organic matter participates in haze formation as well.
Some approaches are known to overcome the haze problem. For example, with respect to the growth defects formed on the photomask upon long-term irradiation of ArF excimer laser light, JP-A 2008-276002 describes that the photomask must be cleaned at a predetermined stage before the photomask can be continuously used. Also JP-A 2010-156880 discloses that haze formation can be inhibited by oxidative treatment of a surface of a photomask blank.
Nevertheless, as the dose of ArF excimer laser light irradiated for pattern transfer increases, the photomask is given damages other than haze. It was found that the line width of the mask pattern changes in accordance with the cumulative irradiation energy dose. See Thomas Faure et al., “Characterization of binary mask and attenuated phase shift mask blanks for 32 nm mask fabrication,” Proc. of SPIE vol. 7122, pp 712209-1 to 712209-12. This problem is that as the cumulative irradiation energy dose increases during long-term irradiation of ArF excimer laser light, a layer of a substance which is considered to be an oxide of the pattern material grows outside the film pattern, whereby the pattern width changes. It is reported that the mask once damaged cannot be restored by cleaning with SC-1 (aqueous ammonia/aqueous hydrogen peroxide) as used in the above-mentioned haze removal or with sulfuric acid/aqueous hydrogen peroxide. It is believed that the damage source is utterly different.
The above article points out that upon exposure of a circuit pattern through a halftone phase shift mask which is the mask technology useful in expanding the depth of focus, substantial degradation is induced by pattern size variation resulting from alternation of a transition metal/silicon base material film such as MoSi base material film by irradiation of ArF excimer laser light (this degradation is referred to as “pattern size variation degradation”). Then, in order to use an expensive photomask over a long period of time, it is necessary to address the pattern size variation degradation by irradiation of ArF excimer laser light.